1. Field of the Invention
An aspect of the present invention to be disclosed relates to a semiconductor device having a wireless communication function (also referred to as an ID tag).
2. Description of the Related Art
In recent years, an RFID system where data are exchanged between an ID tag (also referred to as an RFID tag, an IC tag, an electronic tag, or a wireless tag) and a reader/writer (or a reader) has been popular. Since data are exchanged using an antenna provided for each of an ID tag and a reader/writer in this RFID system, communication is possible even when the ID tag is set apart from the reader/writer by several centimeters to several meters. Moreover, due to an advantage of high resistance against dust, static electricity, and the like, the RFID system has come to be used for a variety of fields including production management in factories, logistics, access control, and the like.
In a case of data communication in this RFID system, the resonant frequency of the antennas of both the ID tag and the reader/writer needs to correspond to the transmitted carrier frequency with a certain degree of accuracy. Here, the resonant frequency f of the antenna is expressed by the following Formula (1) where L is the inductance of an antenna coil and C is the capacitance of a capacitor.f=1/2π√(LC)  (Formula 1)According to Formula 1, the resonant frequency f can be adjusted to a desired value by increasing or decreasing the inductance L of the antenna coil or the capacitance C of the capacitor (see Patent Document 1).
On the contrary, in a case where the resonant frequency f is determined in advance, the product of the inductance L of the antenna coil and the capacitance C of the capacitor needs to be set at a certain value.
The inductance L is proportional to the square of the number of windings of the coil NL and the cross-sectional area thereof SL, and is inversely proportional to the length dL of the coil in its axial direction. In other words, it can be said that the inductance L largely depends on the number of windings of the coil NL. However, once the area of the ID tag is determined, the area occupied by the antenna coil is also determined. That is to say, when the area occupied by the antenna coil is determined, the number of windings of the coil NL is also determined necessarily. Therefore, it is difficult to adjust the resonant frequency f to a desired value by the number of windings of the coil NL.
Meanwhile, the capacitance C of the capacitor is proportional to the area SC of an electrode, and is inversely proportional to the distance dC between the electrodes.
As the capacitor of the ID tag, for example, a parallel-plate capacitor in which each of a top surface and a bottom surface of a dielectric body is provided with an electrode formed using a parallel-plate conductive material is given (see Patent Document 2).